As the trend to weight reduction and higher performance of cars has grown, the strength of springs used for cars has been enhanced and, as a result, a high-strength steel having a tensile strength exceeding 1,600 MPa after heat treatment is being used for springs. Recently, a steel having a tensile strength exceeding 1,900 MPa has been used for this application.
There are two methods in manufacturing coiled springs using a steel: the hot coiling method wherein a steel is heated to the austenitizing temperature range, coiled and then quenched and tempered; and the cold coiling method wherein a high-strength steel wire quenched and tempered beforehand is coiled in a cold state. In either case, the fundamental strength of a spring is determined by quenching and tempering and, for this reason, the design of a chemical composition in consideration of the properties after quenching and tempering is an important factor in manufacturing a spring steel.
Specifically, according to Japanese Unexamined Patent Publication No. S57-32353, hardenability is improved by adding V, Nb, Mo and other elements, and setting resistance is improved by forming fine carbides precipitated during tempering and thereby restricting the movement of dislocations.
Among the aforementioned two methods in manufacturing coiled steel springs, namely, the hot coiling method wherein a steel is heated to the austenitizing temperature range, coiled and then quenched and tempered and the cold coiling method wherein a high-strength steel wire quenched and tempered beforehand is coiled in a cold state, in the case of the cold coiling method, an oil quenching and tempering treatment or a high frequency heat treatment, wherein rapid heating and rapid cooling can be applied during the production of steel wires, is employed, and it is possible to make the size of prior austenite grains in a spring steel material fine, and thus springs excellent in fracture resistance can be manufactured. In addition, as an installation such as a heating furnace or the like can be simplified in a spring manufacturing line, the cold coiling method has the advantage of reducing the equipment cost incurred by a spring manufacturer or the like. For this reason, the spring manufacturing by the cold coiling method is common.
However, as the strength of a steel wire for cold-coiled springs has increased, the steel wire tends to break during the cold coiling and it often becomes impossible to form the steel wire into the shape of a spring. Therefore, it has not always been possible to enjoy a high strength and a good workability at the same time and, for this reason, an industrially disadvantageous manufacturing method has had to be employed for the coiling work. In case of manufacturing a valve spring, though, usually, a steel wire is subjected to an on-line quenching and tempering treatment, namely an oil quenching and tempering treatment, and then coiled in a cold state, a method is employed wherein a steel wire is heated to a temperature at which it is easily deformed in order to prevent the steel wires from breaking during the coiling and, thereafter, subjecting it to a tempering treatment in order to obtain a high strength as disclosed, for instance, in Japanese Unexamined Patent Publication No. H05-179348 wherein a steel wire is heated to 900 to 1,050° C., coiled and then tempered at 425 to 550° C. However, the heating before coiling and the tempering after coiling sometimes cause a dimensional fluctuation of a product spring depending on the condition of heat treatment and/or a remarkable deterioration of treatment efficiency, and, for this reason, a spring manufactured by this method is inferior to that manufactured by the cold coiling method in terms of cost and dimensional accuracy.
In the production processes of a steel, the steel is repeatedly subjected to heating and cooling several times, for example in the processes of converter refining, casting, billet rolling and wire rod rolling. During these processes, carbide forming elements such as Cr, V, Nb and Mo contained in a steel harden the steel and, at the same time, they are likely to be retained in the steel in the form of coarse carbides. In particular, when a high strength exceeding 1,900 MPa in terms of tensile strength is expected, the addition amount of these alloying elements tends to increase and the amount of retained carbides increases accordingly. In view of the situation, Japanese Unexamined Patent Publication No. H11-6033 and some others pay attention to the carbides of Cr, V, Nb, Mo, etc. (hereinafter referred to as “alloy carbides”), and propose to regulate the grain size of the alloy carbides. However, the behavior that actually determines the strength of a steel is not the behavior of the fine carbides of these elements, but the behavior of carbides mainly composed of cementite, namely iron carbides, (hereinafter referred to as “cementite carbides”), and, therefore, the control of the very cementite is important for a spring steel.
With regard to the grain size of alloy carbides, proposals that pay attention to the average grain size of the carbides of Nb and V are advanced, for example, in Japanese Unexamined Patent Publication No. H10-251804. However, in this prior art, there is a description of apprehending that an abnormal structure is generated by cooling water during rolling (paragraph 0015), and the prior art substantially recommends dry rolling.
However, dry rolling, which is distinctly different from normal rolling, is an unstable operation in industrial practices and the prior art suggests that, even when the average grain size is controlled by dry rolling, if unevenness occurs in the peripheral matrix structure, rolling troubles are caused, and that, as a consequence, the control alone of the average grain size of the alloy carbides such as the carbides of V and Nb is not sufficient, industrially.